A) Field of the Invention
The present invention relates to a solid state image pickup device, and more particularly to a solid state image pickup device in which signal charges are accumulated in a number of photodiodes formed on a semiconductor substrate and read to output an image signal.
B) Description of the Related Art
FIG. 9 is a block diagram showing the structure of a charge coupled device (CCD) type solid state image pickup device 51 of a known three-phase drive type (φ1 to φ3).
The solid state image pickup device 51 has a number of photodiodes 62 disposed in a square lattice shape in a light receiving area (image area) 52 and a CCD type vertical charge transfer path (VCCD) 64 disposed for each photodiode column. VCCD 64 has three polysilicon layers and provides three polysilicon electrodes for each photodiode (photodiode row).
Along one side of the light receiving area 52, a CCD type horizontal charge transfer path (HCCD) 53 is disposed and connected to one ends of VCCDs 64. At the succeeding stage of HCCD 53, an output amplifier 55 is connected which is made of a high speed analog amplifier.
Signal charges accumulated in the photodiodes 62 are read to VCCDs 64 at the same time and thereafter sequentially transferred along a vertical direction in VCCDs 64 by a three-phase drive method using drive signals φ1, φ2 and φ3. The signal charges for each row transferred in VCCDs 64 are transferred to HCCD 53. The signal charges in HCCD 53 are transferred along a horizontal direction by a two-phase drive method using drive signals H1 and H2 and the charges for each pixel are transferred to the output amplifier 55.
The output amplifier 55 is made of, for example, a charge/voltage (Q/V) converter unit comprising a floating diffusion amplifier (FDA) and a source follower circuit. An image signal amplified by the output amplifier 55 is output to an external analog signal processing circuit. A horizontal read clock is generally 14 MHz or higher. Higher drive is requested because of a higher density of pixels.
FIG. 10 is a block diagram showing the structure of a known MOS type solid state image pickup device. The MOS type solid state image pickup device 71 has: a light receiving area 72 in which a number of pixels 82 are disposed, each pixel including a photodiode and a charge read circuit for reading charges from the photodiode; a noise eliminating circuit (sample/hold circuit) 73 provided for each column of the pixels 82; an A/D converter circuit (comparator, data latch) 74 provided for each pixel column; a horizontal read circuit 75 including a data register and a horizontal scan circuit used in common by respective columns; and the like.
Charges accumulated in each pixel are read by the charge read circuit and supplied to the noise eliminating circuit 73 and A/D converter 74 via a wiring line to be converted into a digital signal. The digital signals for each row are serially read to an external by the horizontal read circuit 75. If a mechanical shutter does not exist, it is necessary to read an image signal from the image pickup device to the external as fast as possible in order to prevent unnecessary optical signals or dark current from mixing with the image signal. A horizontal read clock is generally 14 MHz or higher. Higher drive is requested because of a higher density of pixels.
When one frame (one full screen image or a partial area of the one full screen image) is taken with a solid state image pickup device, it is necessary to read signals of all pixels from the solid state image pickup device in a limited period of time. It is therefore inevitable to drive the solid state image pickup device at high speed, resulting in an increase of noises and a consumption power.
If a MOS type solid state image pickup device is used, an electronic shutter function is difficult to be realized. If a subject moves in the predetermine period of time taken to read all pixel data, the image flows. Although the motion of a subject can be suppressed small by reading all pixel data at high speed, the exposure time becomes short, which may lower the exposure amount. Also, the operation speed of the horizontal read circuit becomes high and a consumption power increases.